摘要： Metastable crystal phases promise new functionalities by providing access to materials properties beyond those of thermodynamically stable crystalline solids. While realizing metastable phases in the bulk requires non-equilibrium processes such as rapid quenching or high pressures, size effects at the nanometer scale can promote their formation and stabilization during mild processing. Here we demonstrate the controlled formation of axial heterostructures of alternating stable semiconductor (Ge) and metastable metallic (AuGe) segments via post-growth processing, by decorating Ge nanowires with Au, encapsulating them in graphitic carbon shells, annealing at moderate temperatures to induce alloying of Au and Ge in the melt, followed by slow cooling and crystallization. The process is extended to form heterostructures of binary AgGe and ternary AuAgGe alloy segments in Ge nanowires. The metallic alloys adopt different metastable crystal phases that are selected via the nanowire diameter. This phase selection, explained by a size-dependent solubility of Ge in liquid AuGe (and AgGe) and distinct structural motifs in Ge-rich AuGe melts, allows establishing a previously unknown metastable γ-AgGe metastable in thin axially segmented Ge-AgGe nanowires. The findings demonstrate an avenue for synthesizing axial heterostructures of stable and metastable solid phases in nanowires.